In the present study earlier observations of a dual role of the immune response against lymphocytic choriomeningitis (LCM) virus were confirmed and extended. At different times after intracerebral (i.c.) inoculation of the virus, groups of immunosuppressed recipients were transplanted with primary effector cells or memory cells, and the mortality was recorded. The brains and blood of untransplanted immunosuppressed recipients were titrated at intervals after the i.c. challenge, and so were the brains of mice receiving peak primary effector cells at various times relative to the i.c. challenge. The results showed a definite correlation between the brain virus titre at the time of the immune attack and the clinical outcome of the LCM infection. Furthermore, the results indicated that the extent of the extraneural infection is not unimportant but may affect the outcome of the infection through competition for the effector cells. The possible implications of these findings are discussed.